Auxiliary winding ground fault detection for isolated dc/dc converter

ABSTRACT

A flyback converter is provided with a controller that is configured to analyze the reflected feedback voltage waveforms to determine the presence of a ground connection fault for the auxiliary winding.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/583,800, filed May 1, 2017, which is a continuation of InternationalApplication No. PCT/US2015/067166, filed Dec. 21, 2015, which claims thebenefit of U.S. Provisional Patent Application No. 62/146,174, filedApr. 10, 2015, all of which are hereby incorporated by reference intheir entirety.

TECHNICAL FIELD

This application relates to switching power converters, and moreparticularly to the regulation of the power supply voltage for aswitching power supply controller.

BACKGROUND

The explosive growth in mobile electronic devices such as smartphonesand tablets creates an increasing need in the art for compact andefficient switching power converters so that users may recharge thesedevices. A flyback switching power converter is typically provided witha mobile device as its transformer provides safe isolation from AChousehold current. This isolation introduces a problem in that the powerswitching occurs at the primary side of the transformer but the load ison the secondary side. The power switching modulation for a flybackconverter requires knowledge of the output voltage on the secondary sideof the transformer. Such feedback can be obtained through opto-isolatorsbridging from the secondary side to the primary side but this adds tocost and control complexity. Thus, primary-only feedback techniques havebeen developed that use the reflected voltage on the primary side of thetransformer in each switching cycle.

In a switching cycle for a flyback converter, the secondary current (thecurrent in the secondary winding of the transformer) pulses high afterthe primary-side power switch is cycled off. The secondary current thenramps down to zero as power is delivered to the load. The delay betweenthe power switch off time and the secondary current ramping to zero isdenoted as the transformer reset time (Trst). The reflected voltage onthe primary winding at the transformer reset time is proportional to theoutput voltage because there is no diode drop voltage on the secondaryside as the secondary current has ceased flowing. The reflected voltageat the transformer reset time is thus directly proportional to theoutput voltage based upon the turn ratio in the transformer and otherfactors. Primary-only feedback techniques sample this reflected voltagethrough an auxiliary winding to efficiently modulate the power switchingand thus modulate the output voltage.

Although primary-only feedback techniques reduce complexity and cost,the associated transformer is relatively heavy compared to otherboard-mounted components such as integrated circuits. In particular, thetransformer is commonly interconnected to its circuit board through theuse of solder. Modern recycling standards typically require the use oflead-free solder, which is relatively brittle and thus prone tocracking. The resulting failure of the solder interconnect may occurwith regard to the coupling to either the primary or second windings.Such failures will render the resulting flyback unusable but the outputvoltage will never be driven too high as a result. In contrast, if theauxiliary winding's interconnects fail, a reflected voltage will stillappear across the auxiliary winding due to trace inductive, resistive,and capacitive (LRC) effects despite the open circuit fault. The powercontroller will thus react to this reflected voltage and continue tocycle the primary winding's power switch. As a result, the outputvoltage may be driven to dangerously-high levels due to the interconnectfault for the auxiliary winding, which results in damage to theassociated load. But conventional power controllers have no way ofdetermining that the auxiliary winding interconnects have failed.

Accordingly, there is a need in the art for improved fault detection forprimary-only-feedback-regulated flyback converters.

SUMMARY

A flyback converter is provided with a controller that is configured toanalyze the reflected feedback voltage waveforms to determine thepresence of a ground connection fault for the auxiliary winding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a flyback converter including a controllerconfigured to analyze the reflected voltage waveform to detect a faultcondition with regard to the auxiliary winding interconnects inaccordance with an embodiment of the disclosure.

FIG. 1B illustrates the flyback converter of FIG. 1 after occurrence ofa ground disconnect for the auxiliary winding.

FIG. 2A illustrates two switch cycles for the flyback converter of FIG.1B.

FIG. 2B illustrates the resulting secondary current waveforms inresponse to the switch cycles of FIG. 2A.

FIG. 2C illustrates the resulting reflected feedback waveforms inresponse to the switch cycles of FIG. 1B for both the presence andabsence of an auxiliary winding ground disconnect.

FIG. 3 illustrates an example controller in accordance with anembodiment of the disclosure.

FIG. 4A illustrates the voltage thresholds as compared to the reflectedfeedback voltage waveform for the controller of FIG. 3.

FIG. 4B illustrates the comparator output signals for a normal reflectedfeedback voltage waveform.

FIG. 4C illustrates the comparator output signals for a faulty reflectedfeedback voltage waveform due to the presence of an auxiliary windingground disconnect.

FIG. 5A illustrates a voltage threshold as compared to the reflectedfeedback voltage waveform for a single-comparator controller embodiment.

FIG. 5B illustrates the comparator output signal for a normal reflectedfeedback voltage waveform in a single-comparator controller embodiment.

FIG. 5C illustrates the comparator output signal for a faulty reflectedfeedback voltage waveform in a single-comparator controller embodiment.

Embodiments of the present disclosure and their advantages are bestunderstood by referring to the detailed description that follows. Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures.

DETAILED DESCRIPTION

To address the need in the art for improved fault detection, a flybackconverter controller is provided that is configured to analyze thereflected voltage waveform to detect an interconnect failure for theauxiliary winding. Should the controller detect a failure, it may thenreset the switching power converter into an idle mode so that the outputvoltage is not driven out of regulation. In addition, a signal may begenerated to alert the user of the fault condition. These advantageousfeatures may be better appreciated with regard to the following exampleembodiments.

Turning now to the drawings, FIG. 1A illustrates an example flybackconverter 100 including a controller U1 configured to practice thedefault detection method disclosed herein. A rectified input voltage(V_IN) drives a primary winding T1 of a transformer 105 when controllerU1 switches on a power switch. In converter 100, the power switch is aMOSFET S1 power switch but it will be appreciated that bipolar junctiontransistor (BJT) switches may be used in alternative embodiments. Tocycle the power switch on, controller U1 charges a gate of power switchtransistor S1 to switch it fully on. Based upon the input voltage V_INand a magnetizing inductance for the transformer, a primary windingcurrent in primary winding T1 then ramps up from zero to a peak currentvalue, whereupon controller U1 switches off power switch transistor Sito complete a switching cycle.

Controller U1 controls the peak primary current responsive to a feedback(V_FB) voltage derived from a reflected voltage on an auxiliary winding(T1_AUX). When controller U1 switches off power switch transistor S1, arectifying diode D1 coupled to a second winding S1 of transformer 105becomes forward biased such that the stored magnetic energy intransformer 105 is delivered as an output voltage (V_OUT) across a load110 as filtered by a load capacitor C1. It will be appreciated thatrectifying diode D1 may be replaced by a synchronous rectification (SR)switch in alternative embodiments. This delivery of energy to load 110produces a reflected voltage on the auxiliary winding that is a functionof the voltage drop across the diode D1 and the output voltage V_OUT. Asthis energy delivery is depleted, a secondary current in the secondarywinding S1 will drop to zero such that there is no voltage drop acrossdiode D1, whereupon the reflected voltage across the auxiliary windingis directly proportional to the output voltage V_OUT. This time isdenoted as the transformer reset time (Trst) and represents the idealtime to sample the reflected voltage V_FB to obtain an accurate estimateof the output voltage V_OUT.

The feedback voltage V_FB is just one parameter that may be used in theprimary-only feedback implemented by controller U1. For example, theprimary winding current may be sampled through a sense resistor (notillustrated) to produce an I_(sense) voltage that represents the primarywinding current amplitude. Controller U1 may use the rate of change ofthe primary winding current as determined through the I_(sense) voltageto indirectly measure the input voltage V_IN. This is quite advantageousas controller 105 may then determine the input voltage V_IN withoutrequiring an additional input pin. In this fashion, controller 105 mayprocess V_FB and I_(sense) from a previous pulse to determine thedesired peak primary winding current in the subsequent pulse on apulse-by-pulse basis.

Such primary-only feedback control of the output voltage V_OUT bycontroller U1 is conventional. However, this conventional primary-onlyfeedback control becomes problematic should the auxiliary winding nolonger couple to ground as shown in FIG. 1B for flyback converter 100.In particular, an interconnect 120 coupling the auxiliary winding T1_AUXto ground has failed such as through a crack or other defect. Despitethis failure, the auxiliary winding may still couple to ground throughstray inductive, resistive, and capacitive elements as represented bycapacitor 125. The switch state for MOSFET S1, the waveform for thesecondary winding current, and the auxiliary voltage waveform are shownin FIG. 2A, FIG. 2B, and FIG. 2C, respectively. As shown in FIG. 2A, thepower switch such as MOSFET S1 is pulsed on to drive current through theprimary winding. When S1 is turned off, the secondary current(I_SECONDARY) is pulsed high to then linearly ramp down to zero as shownin FIG. 2B. During a normal mode of operation (no interconnectfailures), the resulting reflected voltage V_FB is as shown by dottedline 200. There are two cycles of MOSFET S1 and thus two correspondingcycles for reflected voltage 200 (as used herein, the term “cycle” isused to refer to the voltage waveform that is produced responsive to oneswitching cycle for the power switch transistor).

Should the auxiliary winding become disconnected due to interconnectfault 120, faulty reflected voltage cycles 205 are produced. Eachpulsing of switch S1 produces a corresponding cycle of the faultyreflected voltage 205. To obtain an estimate of the output voltage in aprimary-only feedback architecture, a conventional controller wouldsample faulty reflected voltage cycles 205 such as at times T1 and T2.Due to the auxiliary winding fault, the sampled feedback voltage (V_FB)for faulty cycles 205 will be considerably lower than the sampled valuesfor normal cycles 200. The difference between the sampled voltage and athreshold voltage is used by primary-only-feedback controllers tocalculate the desired peak primary current for the subsequent switchingcycle (or cycles). Faulty cycles 205 result in the controller drivingexcessive peak primary currents due to the abnormally-low values for thesamples of the reflected feedback voltage (V_FB). A conventionalcontroller is thus “fooled” by aberrant reflected voltage cycles 205 soas to drive the output voltage out of regulation higher than the desiredlevel. The resulting increased output voltage may damage sensitive loadcircuits that cannot accommodate such relatively-high voltage levels.

To prevent the output voltage from being driven out of regulation due toa ground disconnection of the auxiliary winding, controller U1 isconfigured to detect abnormally-slow declines in the reflected voltagewaveforms following the switch off time. For example, controller U1 mayinclude a pair of comparators 300 and 305 as shown in FIG. 3. Comparator300 compares the feedback voltage to a relatively high threshold voltage(REF_A). In contrast comparator 305 compares the feedback voltage to alower threshold voltage (REF_B). Both the threshold voltages are chosensuch that they are lower than the expected feedback voltage at thetransformer reset time as shown in FIG. 4A. Normal reflected feedbackvoltage cycle 200 will thus only fall below the threshold voltage afterthe transformer reset time (Trst). In normal reflected voltage cycle200, the voltage decrease is very rapid after the transformer resettime. In contrast, although faulty reflected feedback voltage cycle 205begins to decline much earlier, it declines at a slower rate as shown inFIG. 4A. Comparators 300 and 305 are configured to assert their outputsignals when the feedback voltage is greater than their respectivethresholds but it will be appreciated that a complementary configurationin which comparators 300 and 305 assert their output signals only whenthe feedback voltage is lower than their threshold voltages may be usedin alternative embodiments.

The resulting comparator output signals are shown in FIG. 4B for normalreflected feedback voltage cycle 200. Due to the rapid decline in thefeedback voltage subsequent to the transformer reset time, thedifference between the time when comparator 300 pulls its output signallow as compared to when comparator 305 pulls its output signal low isrelatively small—e.g., 100 nanoseconds. In contrast, the comparatoroutput signals for faulty reflected feedback voltage cycle 205 are shownin FIG. 4C. Due to the relatively slow decline in the feedback voltagewhen the auxiliary winding is disconnected from ground, the differencein time between the falling edges for the comparator output signals isrelatively large—e.g., a microsecond or more. Controller U1 may thusinclude a timing analysis circuit 310 as shown in FIG. 3 that comparesthe period between the falling edges for the comparator output signalsfrom comparators 300 and 305 to a threshold level (e.g., 500nanoseconds). Should the period between the falling edges be less thanthe threshold value, controller U1 continues in a normal mode ofoperation. Conversely, should the period exceed the threshold value,controller U1 may trigger a reset to prevent the load from being drivenout of regulation. In addition, controller U1 may alert the userregarding the fault condition being detected.

An alternative embodiment, controller U1 may determine a fault usingjust one comparator such as comparator 300. Its reference voltage(REF_A) would be adjusted as shown in FIG. 5A such that it will becrossed by the ringing of the feedback voltage 200 that occurs after thesteep decline following the transformer reset time. The ringing isfairly regular or sinusoidal such that the rising edges in thecomparator output signal will have a fairly-constant separation as shownin FIG. 5B following normal cycle 200. The falling edges also have thisregular separation. But faulty reflected feedback cycle 205 will firsthave an abnormally-long delay between the initial falling edge in thecomparator output signal as compared to the subsequent falling edges.Timing analysis circuit 310 in a one-comparator-embodiment may thus beconfigured to compare the initial blanking time for the comparatoroutput signal to a threshold value. Should this threshold value beexceed, controller U1 asserts the reset signal and/or signals the userthat a fault condition exists. This is quite advantageous as theresulting modification to controller U1 is quite compact as it involvesjust one or two comparators and some associated timing logic yet thedangers of too-high output voltage due to auxiliary winding disconnectsare eliminated.

As those of some skill in this art will by now appreciate and dependingon the particular application at hand, many modifications, substitutionsand variations can be made in and to the materials, apparatus,configurations and methods of use of the devices of the presentdisclosure without departing from the scope thereof. For example,alternative detectors as compared to the use of a comparator may be usedwith regard to determining if the power switch should be cycled tobolster the controller power supply voltage. In light of this, the scopeof the present disclosure should not be limited to that of theparticular embodiments illustrated and described herein, as they aremerely by way of some examples thereof, but rather, should be fullycommensurate with that of the claims appended hereafter and theirfunctional equivalents.

We claim:
 1. A method, comprising: cycling a power switch coupled to aprimary winding to generate an output voltage at a load coupled to asecond winding and to generate a reflected feedback voltage on a primarywinding; after an off time for the power switch in each cycle of thepower switch, comparing a rate of decline for the reflected feedbackvoltage to a threshold value; and ceasing the cycling of the powerswitch responsive to the comparison indicating that the rate of declineexceeded the threshold value.
 2. The method of claim 1, furthercomprising: using at least one comparator to compare the rate of declinefor the reflected feedback voltage to the threshold value.
 3. The methodof claim 1, wherein comparing the rate of decline for the reflectedthreshold voltage comprises: detecting a first time when the reflectedfeedback voltage declines below a first threshold value; and detecting asecond time when the reflected feedback voltage declines below a secondthreshold value that is lower than the first threshold value.
 4. Themethod of claim 3, further comprising: determining that the rate ofdecline for the reflected feedback voltage has exceeded the thresholdvalue by determining that the first time and the second time areseparated too widely.
 5. A method, comprising: cycling a power switchcoupled to a primary winding to generate an output voltage at a loadcoupled to a second winding and to generate a reflected feedback voltageon a primary winding; after each an off time of the power switch in eachcycle of the power switch, comparing the reflected feedback voltage to athreshold value; and ceasing the cycling of the power switch responsiveto the comparison indicating a ringing of the reflected feedback voltagewas not periodic.
 6. The method of claim 5, further comprisingcontinuing the cycling of the power switch responsive to the comparisonindicating that the ringing of the reflected feedback voltage wasperiodic.
 7. A controller for a switching power converter, comprising: acomparator configured to compare a feedback voltage for the switchingpower converter to a threshold voltage; and a timing analysis circuitconfigured to measure a delay between a first time when the comparatordetects that the feedback voltage is less than the threshold voltage anda subsequent second time when the comparator detects that the feedbackvoltage is less than the threshold voltage, wherein the timing analysiscircuit is further configured to detect a ground fault for the switchingpower converter responsive to the delay being greater than a thresholddelay.
 8. The controller of claim 7, wherein the controller is furtherconfigured to stop cycling a power switch responsive to the delay beinggreater than the threshold delay.